1. Field of the Invention
The present invention relates to a method of making a thin film transistor (TFT) for driving a liquid crystal display (LCD), and more particularly to a method of making a TFT adequate to achieve picture quality of high resolution by improving recrystalization uniformity of the poly silicon layer obtained by laser beam processing.
2. Description of the Prior Art
A TFT LCD has the construction that high-integrated TFTs for driving an LCD are added thereon and has been developed and used as a main image device for portable televisions, wall-hanging type televisions, notebook type computers, etc., since it can provide high resolution picture quality.
FIG. 1 shows the construction of an amorphous silicon TFT, which is the kind of TFT used for the above-mentioned TFT LCD. In making such amorphous silicon TFT, a metal such as Cr, Ta, Ta alloy, Al, etc. is evaporated on a glass substrate 21 and then patterned to form a plurality of gates 22.
An insulating layer 23 is grown on the gate-formed glass substrate 21, and then amorphous silicon 24 is evaporated thereon and patterned.
On the patterned amorphous silicon 24, an n.sup.+ -type amorphous silicon 25 is grown and patterned and on the patterned n.sup.+ -type amorphous silicon 25, electrode metal 26 is coated, resulting in completion of amorphous silicon TFTs.
The amorphous silicon TFTs constructed as above can be used for driving the LCD as mentioned above. That is, as shown in FIG. 4, when voltage is applied to a gate electrode a, an electric signal is transmitted from source electrode b to drain electrode c, causing LCD d to be charged and driven. The characteristics of a TFT are dominated by the characteristics of amorphous silicon layer 24.
Usually, the field effect mobility of an amorphous silicon TFT is in the range of 0.5.sup..about. 1.0 cm.sup.2 /visec. And it has been reported that a TFT having field effect mobility of 4.sup..about. 5 Cm.sup.2 /visec is in development.
However, an amorphous silicon TFT has a relatively low field effect mobility as mentioned above, so that it is inadequate to drive an LCD having a large screen or for a high picture quality.
Furthermore, all manufacturing processes must be carried out below the temperature of about 600.degree. C. because of the glass substrate.
Meanwhile, to improve the field effect mobility of a TFT, a method of making a TFT having a polysilicon layer recrystallized by a laser beam process has been proposed.
According to the above method, the temperature distribution is adjusted by forming an insulating layer having the configuration shown in FIGS. 2A and 2B or by adjusting the shape of laser beam as in FIG. 3 without the insulating layer in order to effect selective crystallization of the amorphous silicon layer.
In case selective crystallization is carried out by adjusting the temperature distributing using the thickness of the insulation layer, as shown in FIG. 2A, a crystal core is formed and grown in the part of the amorphous silicon layer placed under the convex part of the insulation layer owing to the warmth retention effect of the convex part of the insulating layer, thereby selective crystal-growing is accomplished and optical defects are concentrated in the concave part of the insulation layer, or as shown in FIG. 2B, optical defects are formed in the part of the amorphous silicon layer above the concave part of the insulation layer owing to the heat sink function of the convex part of the insulation layer under the amorphous silicon layer.
As shown in FIG. 3, in case crystallization is carried out by adjusting the beam shape by means of an optical system including a filter, a mode selector, a beam overlapper, a splitter, a lens, etc., a crystal core is formed along the M--M line and grows to either side of the beam, so that the active region of the TFT is positioned in the central part of the beam.
However, the method of making a TFT described as above has the disadvantage that it is required to form the insulating layer and perform a complicated etching process to make the insulation layer uneven prior to laser beam scanning as shown in FIG. 2.
Also, in case the insulation layer is formed too thinly, adjustment of the temperature distribution is incomplete, and thus, the process is delayed until the layer is formed with insulating thickness of more than 1 mm.
Furthermore, as shown in FIG. 3, in using an optical system including a filter, an aperture, etc., the degree of crystallization may be weakened owing to reduction of the power of the laser beam. And since the size of the beam is relatively small (.phi.50 .mu.m.sup..about. .phi.10 mm) in making a TFT for a large-scaled LCD, laser beam processing time is prolonged and the number of overlapping parts (6.sup..about. 8%) between beam scan lines increases, as well as making the position adjustment among the respective TFTs and the overlapping parts difficult.